Add IHtmlContentBuilder interface

Common interface for things that allow appending content (TagBuilder,
BufferedHtmlContent). We want to be able to expose this from APIs for
users to add content.

See discussion: https://github.com/aspnet/Mvc/issues/3087

src/Microsoft.AspNet.Html.Abstractions/IHtmlContentBuilder.cs

@@ -0,0 +1,31 @@
+// Copyright (c) .NET Foundation. All rights reserved.
+// Licensed under the Apache License, Version 2.0. See License.txt in the project root for license information.
+
+namespace Microsoft.AspNet.Html.Abstractions
+{
+    /// <summary>
+    /// A builder for HTML content.
+    /// </summary>
+    public interface IHtmlContentBuilder : IHtmlContent
+    {
+        /// <summary>
+        /// Appends an <see cref="IHtmlContent"/> instance.
+        /// </summary>
+        /// <param name="content">The <see cref="IHtmlContent"/> to append.</param>
+        /// <returns>The <see cref="IHtmlContentBuilder"/>.</returns>
+        IHtmlContentBuilder Append(IHtmlContent content);
+
+        /// <summary>
+        /// Appends a <see cref="string"/> value. The value is treated as unencoded as-provided, and will be HTML
+        /// encoded before writing to output.
+        /// </summary>
+        /// <param name="content">The <see cref="string"/> to append.</param>
+        /// <returns>The <see cref="IHtmlContentBuilder"/>.</returns>
+        IHtmlContentBuilder Append(string unencoded);
+
+        /// <summary>
+        /// Clears the content.
+        /// </summary>
+        void Clear();
+    }
+}